1. Field of the Invention
The present invention relates to image forming apparatuses using electrophotographic technology, more particularly to an image forming apparatus including a transfer member made with an ion conductive material.
2. Description of Related Art
The electrographic technology renders it possible to readily obtain a high-quality image and therefore is widely used in image forming apparatuses such as printers. As is well-known, the electrographic technology incorporates a charging step, an exposing step, a developing step, a transferring step, a cleaning step, and a fixing step. Among these steps, in the transferring step, a toner image formed on a photoreceptor drum is transferred either using an intermediate transfer belt or directly onto a print medium, such as a sheet of paper or an overhead projector (OHP) sheet. In the transferring step, a transfer roller is pressed against an image carrier, such as the photoreceptor drum or the intermediate transfer belt, forming a transfer nip therebetween. When the print medium passes through the transfer nip, a transfer bias voltage is applied to the transfer roller, so that a charge having an opposite polarity to toner is provided to the back face of the print medium. Thus, the toner image is transferred from the image carrier onto the print medium.
Some transfer rollers have a layer made of an ion conductive material (e.g., a rubber layer). Such a transfer roller passes current by means of ions in the layer carrying electrons. However, during a print operation, if a transfer bias voltage of the same polarity continues to be applied to the transfer roller, the ions are unevenly distributed in the transfer roller. As a result, the ions that carry electrons decrease in number compared to the initial state, so that the resistance of the transfer roller rises. The degree of the uneven ion distribution increases as the amount of current running through the transfer roller, which is determined by the value of current and the time of application, increases. In other words, the resistance of the transfer roller increases proportionally to the increase of the amount of current.
In view of the above, for example, in Japanese Laid-Open Patent Publication No. 2006-163266, once the resistance of the transfer roller has exceeded a threshold, a reverse-bias voltage V2, which has an opposite polarity to the transfer bias voltage used in the transferring step, is applied to the transfer roller. Consequently, the uneven ion distribution in the transfer roller is lessened, resulting in lower resistance of the transfer roller.
Incidentally, the transfer nip includes an area through which the print medium passes (i.e., a passage area) and an area through which no medium passes (i.e., a nip-margin area). Here, the nip-margin area of the transfer roller is not affected by the resistance of the print medium, and therefore, at the initial stage of continuous printing (i.e., serial printing on a plurality of print media), the nip-margin area passes a higher current compared to the passage area. However, the resistance of the ion conductive material rises as the value of current increases, and therefore, the resistance of the nip-margin area rises faster than the resistance of the passage area. In other words, the amount of current in the nip-margin area gradually decreases. As a result, at some point during the continuous printing, the amount of current in the passage area might become excessively high, resulting in a so-called excessive transfer. Here, the excessive transfer refers to a phenomenon where the toner on the image carrier is inversely charged because the current running through the passage area is excessively high relative to the amount of charge in the toner, so that the toner is not properly transferred to the print medium. Such an excessive transfer might lead to print density failure.
However, in Japanese Laid-Open Patent Publication No. 2006-163266, the reverse-bias voltage V2 is applied to the transfer roller depending on the resistance of the entire transfer roller, including a portion on which the print medium is present. In other words, an increase in the current value of the passage area due to an increase in the resistance of the nip-margin area is not taken into consideration. Accordingly, there is a problem where the reverse-bias voltage V2 is not applied at an appropriate time, leading to susceptibility to print density failure.